Blue supergiants in the Pinwheel Galaxy M101: comparison with H II region chemical abundances, spectroscopic distance and an independent determination of the Hubble constant

TL;DR

This study uses blue supergiant stars in M101 to compare stellar and nebular metallicities, determine the galaxy's distance via the FGLR method, and independently estimate the Hubble constant, aligning well with previous measurements.

Contribution

It provides a detailed spectroscopic analysis of blue supergiants in M101, linking stellar metallicities with nebular data, and offers an independent measurement of the Hubble constant using the FGLR method.

Findings

Stellar metallicity decreases from 1.9 Zsun to 0.3 Zsun across M101.

The FGLR distance to M101 is 6.5 Mpc, consistent with other methods.

The Hubble constant derived is 72.5 km/s/Mpc, aligning with previous estimates.

Abstract

We present a quantitative spectroscopic study of 13 blue supergiant stars in the Pinwheel Galaxy M101, based on data obtained with the Low Resolution Imaging Spectrometer available at the Keck I telescope. The average stellar metallicity decreases from ~1.9 Zsun near the center of the galaxy to ~0.3 Zsun at the optical outskirts. The galactocentric radial metallicity gradient is statistically consistent with previous studies of the gas-phase oxygen abundance from H II regions using the direct method. The H II region-based Cepheid metallicities used by Riess et al. in their determination of the Hubble constant H_0 are in substantial agreement with our measurements. The direct method gas-phase metallicities of the 18 star-forming galaxies we have analyzed so far, when adjusted upward for a mean ~0.15 dex oxygen dust depletion factor, are in good agreement with those we infer from the supergiants, over a factor of 50 in metallicity. From the same data, we derive an expression for the metal-dependent depletion of oxygen in photoionized nebulae. Utilizing the flux-weighted gravity - luminosity relationship (FGLR) of blue supergiants, we measure a distance to M101, D=6.5 +\- 0.2 Mpc (m-M = 29.06 +\- 0.08), which is within 1 sigma from determinations based on the tip of the red giant branch and Cepheids. With M101 as a nearby SN Ia host and using the observed standardized B-band magnitude of the supernova, our FGLR distance yields an independent value H_0 = 72.5 +\- 4.6 km/s/Mpc.